傘と提灯は戻らぬつもりで貸せ
(Kasa to chouchin wa modoranu tsumori de kase;
“Lend umbrellas and lanterns with the awareness that they will not return”)

Definition:

People are forgetful. Human nature being what it is, there are certain things that you should expect, such as loaned umbrellas not being returned. You only take out an umbrella when there’s danger of being rained on, and you only take out a lantern when you need to go somewhere after dark, so both are easy to forget about when the sun is out. If you lend one to somebody, they’re likely to stow it away when it’s not needed, and forget about it, and not return it. When it is needed, of course, they’ll want to use it, and tell themselves that they’ll return it afterwards – but you probably won’t be there at the right moment, so they’ll stow it away until they have the chance, and the cycle continues. In short, be prepared to never see it again.

Breakdown:

The topic marker は (wa) tells us that we’re talking about the noun 傘 (kasa), “umbrella” and the noun 提灯 (chouchin), an old-style rounded paper lantern, joining them with と (to), “and.” The comment on this topic begins with intransitive verb 戻る (modoru), “to return,” in imperfective form and with negative suffix ず (zu) in prenominal form as ぬ (nu). This allows it to connect with the noun つもり (tsumori), “intent,” “belief,” which in turn is marked by the particle で (de), in this case, “with.” The verb performed with the belief that the implements won’t return is 貸す (kasu), “to lend,” in imperative form.

Notes:

While my interpretation generalizes the principle for an age in which we tend not to travel by the light of hand-held lanterns, the use of と rather than や suggests an exclusive list rather than two primary examples from an implied expansive list. Whether this is a mere coincidence, an example of usage shifting over time, or a deliberate attempt to separate forgettable things like umbrellas from things that you really shouldn’t forget (like money), is unclear.

Example sentence:

「おい、貸熊、クロックさんが着てるのってお前のセーターじゃないの？」　「うん、でも、別に大丈夫ですよ。傘と提灯と同じように戻らぬつもりで貸したの」　「…なんだよ」　「ううん、ただ、私のセーターだってよく気付きましたね、先輩」

(“Oi, Kashikuma, Kurokku-san ga kiteru notte omae no seetaa ja nai no?” “Un, demo, betsu ni daijoubu desu yo. Kasa to chouchin to onaji you ni modoranu tsumori de kashita no.” “…Nan da yo.” “Uun, tada, watashi no seetaa datte yoku kidzukimashita ne, senpai.”)

[“Hey, Kashikuma, isn’t that your sweater that Clock-san is wearing?”

“Yeah, but it’s fine. I lent it to her knowing that, like an umbrella, it would probably never come back.”

“…What.”

“Nooothing, just, I’m impressed that you noticed it was my sweater, senpai.”]